Magnetic domain structures in thin foils may be revealed in a number of ways in the transmission electron microscope. The most commonly used modes (Fresnel, Foucault, holographic, differential phase contrast and low angle diffraction) are described and intercompared. The Fresnel and Foucault modes are simple to implement but provide a mainly qualitative description of the spatial variation of induction throughout the foil. Quantitative information may be obtained from the other two imaging modes but they require more sophisticated equipment. Electron diffraction is particularly useful for studying periodic magnetic structures. Examples of the use of these, and further specialised techniques, are presented by reference to some applications of current interest within magnetism.

https://iopscience.iop.org/article/10.1088/0022-3727/17/4/003/pdf

The investigation of magnetic domain structures in thin foils by electron microscopy

The recently discovered oscillating exchange effect in iron–chromium multilayers can best be studied on an epitaxial iron sandwich with a wedge shaped chromium interlayer. Domain patterns are analyzed as well as magnetization curves on such samples using magnetooptical techniques. Unusual domains are found in zones around the transitions between ferromagnetic and antiferromagnetic exchange. Also the magnetization curves for larger chromium thickness show some peculiar features. The interpretation of these patterns invokes coupling between the iron layers which favours a right angle between the magnetization directions in both layers. Such a non-collinear interaction can be derived phenomenologi-cally from a biquadratic coupling as introduced earlier in the analysis of spin structures in oxides. A biquadratic coupling can be understood either as a second order Heisenberg or a second order Dzyaloshinskii interaction. In the course of these investigations domain observations proved to be a useful and sensitive method to detect locally the nature of interactions between ferromagnetic layers.



Die kiurzlich entdeckte oszillierende Wechselwirkung in Eisen–Chrom-Vielfachschichten kann am besten an einer epitaxialen Schichtanordnung mit einer keilformigen Chromzwischenschicht studiert werden. Die auf solchen Proben beobachtbaren Domanenstrukturen sowie die Magnetisierungskurven werden mit magnetooptischen Verfahren untersucht. Im Bereich der Ubergange zwischen ferroma-gnetischer und antiferromagnetischer Kopplung werden Zonen ungewohnlicher Muster entdeckt. Auch die Magnetisierungskurven fur grosere Chrom-Schichtdicken zeigten eigentumliche Zuge. Die Beobach-tungen lassen sich erklaren, wenn eine zusatzliche Kopplung der Schichten angenommen wird, die einen rechten Winkel zwischen den Magnetisierungsrichtungen in beiden Schichten begunstigt. Phanomenologisch ergibt sich eine solche nicht-kollineare Kopplung aus der Annahme einer biquadrati-schen Wechselwirkung, wie sie fruher in der Analyse von Spinstrukturen magnetischer Oxide eingefuhrt wurde. Eine biquadratische Kopplung Iast sich entweder als Heisenberg- oder Dzyaioshinskii-Wechselwirkung zweiter Ordnung interpretieren. Domanenbeobachtungen erwiesen sich im Laufe dieser Untersuchung als empfindliche Methode zur Untersuchung des Charakters der Kopplung in Vielfachschichten.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/pssa.2211250225

Domain Observations on FeCrFe Layered Structnres. Evidence for a Biquadratic Coupling Effect

Recently, significant interest has emerged in fabricated systems that mimic the behavior of geometrically frustrated materials. We present the full realization of such an artificial spin ice system on a two-dimensional kagome lattice and we demonstrate rigid adherence to the local ice rule by directly counting individual pseudospins. The resulting spin configurations show not only local ice rules and long-range disorder, but also correlations consistent with spin ice Monte Carlo calculations. Our results suggest that dipolar corrections are significant in this system, as in pyrochlore spin ice, and that they open a door to further studies of frustration in general.

https://terpconnect.umd.edu/~cumings/PDF%20Publications/31.PRL77qi.pdf

Direct observation of the ice rule in an artificial kagome spin ice

Some new developments are reported in the technique of observing magnetization in thin ferromagnetic films by transmission electron microscopy, and the results of some observations are described that contribute to an understanding of the magnetic character of thin films. Two modes of operation of a microscope are explained, one using projector magnification alone, and one using both objective and projector lenses. The intensity distribution images of the magnetization distribution in a thin film is derived from the Lorentz force acting on an electron passing through the film. The intensity distributions for some simple domain walls are computed and compared in various ways with experimental observations. A magnetization ripple model is proposed and justified to explain an intensity texture observed principally when projector magnification alone is used. The texture is shown to be an orthogonal map of the magnetization of the film, and to be of use in interpreting wall structures. High magnification and resolution wall micrographs are obtained using objective magnification, and a direct measurement of the wall thickness and shape can be made in this way. 180° walls in a Fe‐Ni alloy film of about 200 A thickness are found to have a width of 2δ = 1500 A, using the wall model cot−1θ = −sinhx/δ. The effect of finite resolution on the measurements is discussed. It is shown that the magnetization distribution of a wall can be obtained from the measured intensity distribution by inverting the parametric equation solution for the intensity distribution.

Determination of Magnetization Distribution in Thin Films Using Electron Microscopy

It has been shown that isolated magnetic domains in thin platelets (≍2 mils thick) of orthoferrites can be manipulated to perform memory, logic, and transmission functions. The purpose of this paper is to discuss the properties of orthoferrites that make them suitable for magnetic device applications and consider magnetostatic problems relevant to domain structures found to be useful. Included is a brief indication of how memory, logic, and transmission can be accomplished; however, the details will be reserved for a later paper. The stability conditions of a cylindrical domain are discussed in detail and data is reported to support the conclusions. Of particular interest are the sizes of cylindrical domains available in the various orthoferrites. Such data has been taken on five of the fourteen possible orthoferrites and it is found that the thulium orthoferrite, TmFeO 3 , gives the smallest stable domain diameter (2.3 mils) and LuFeO 3 the largest. The stability results lead to a direct method for obtaining σ W , the domain wall energy density. For TmFeO 3 , as an example, σ W = 2.8 ergs/cm2. It is concluded that the orthoferrites are well suited indeed for device applications. Experimentally, 3 mil diameter domains have been manipulated and there is every reason to believe that operation of sub-mil domains will soon be realized.

Properties and device applications of magnetic domains in orthoferrites

Magnetic Domain Observations by Electron Microscopy

A new method for observing full domains in thin magnetic films by electron microscopy is described. Observations are made with standard transmission instruments utilizing an off‐centered objective aperture diaphragm as a knife edge. The method has the high‐resolution advantage that the microscope is focused on the specimen during domain observations. Limitations of the method and comparisons with the previously reported defocusing technique are presented. Applications to the interpretation of complex domain patterns and cross‐tie walls are demonstrated. The observations were made with electrostatic‐focusing microscopes, the AEG‐Zeiss and Trub‐Tauber instruments, which allow the use of full objective power without influencing the magnetization distribution of low coercive force films. A second method using a knife edge is proposed that would potentially permit a simple measurement of the detailed magnetization distribution of a domain wall in a thin film.

Domains in Thin Magnetic Films Observed by Electron Microscopy

This paper discusses some magnetostatic interactions between two thin films that are separated by a small distance between parallel film planes. The external field of a domain wall in one film is calculated using a simple wall model, and the effect of this field on a second adjacent film is determined. The conditions are investigated under which the wall field of the first film together with an external field can cause nucleation of a domain in the second film. The interaction energy between a movable wall in one film and a stationary wall or a linear imperfection in a second adjacent film is calculated using simple models. The equilibrium position of the movable wall as a function of an externally applied field is then determined. Means for electrically modifying the degree of interaction are investigated. Experimental results are presented illustrating the aforementioned interactions. The nucleation of long narrow domains in one film is achieved by the use of a domain wall, a scratch imperfection, or a film edge in a second adjacent film. The influence of a stationary wall in one film on the controlled wall motion of a wall in a second film is demonstrated. A comparison of theory and experiment is made. Mention is made of how the subject interactions might be utilized in new information storage devices.

Magnetostatic Interactions between Thin Magnetic Films

Domain walls in thin films of permalloy were previously found, by Huber et al., to frequently exhibit periodically spaced “cross‐ties.” These wall structures have been studied in some detail in a variety of alloys and the model advanced by Huber et al., largely substantiated. Of some interest is the great sensitivity of the position of normal spin orientation between cross‐ties to externally applied fields. A nonmagnetic overcoating on a thin film, and dark‐field illumination with restricted‐azimuth are methods which are valuable in such observations using the Bitter technique. Other characteristic wall configurations peculiar to thin ferromagnetic films have been observed and models have been devised for the magnetization distributions. Powder patterns have been observed on an iron film at intermediate stages of the slow switching of the film. The powder is seen to collect along lines perpendicular to the applied field prior to the formation of domains. A model based on magnetization buckling is suggeste...

Harrison W. Fuller

Papers

Determination of Magnetization Distribution in Thin Films Using Electron Microscopy

Magnetic Domain Observations by Electron Microscopy

Domains in Thin Magnetic Films Observed by Electron Microscopy

Magnetostatic Interactions between Thin Magnetic Films

Observations Made on Domain Walls in Thin Films